Duplexer with improved transmission/receiving band separation

ABSTRACT

In a duplexer for a wireless communication system that comprises a transmission and a reception band, it is proposed that a split surface wave filter be provided as input and/or output filter, said slit surface wave filter being composed of at least two sub-filters that cover neighboring frequency sub-ranges of the corresponding band and supplementing one another to the overall band. At least two pairs of sub-filters are provided that respectively comprise a filter in the input and out put filter. Switching between the at least two pairs can be carried out with a switch. Given the same duplexer spacing, a higher band spacing that can be realized with SAW filters is created between transmission and reception band or, respectively, the corresponding frequency sub-ranges.

[0001] In wireless communication systems, particularly in mobileradiotelephone systems that do not allow TDD (time domain duplexing),two different frequency bands are usually provided that serve astransmission band and reception band from the point of view of thecommunication participant. A common antenna is used for the transmissionand reception of signals on the communication transmission device,particularly in the mobile radiotelephone device (cell phone). Aduplexer is therefore generally needed for the separation oftransmission and reception signals, this being connected between antennaand transmission and reception path. Such a duplexer is essentiallycomposed of two interconnected filters, namely a RX filter betweenantenna and Rx path (LNA=low noise amplifier) for received signals and aTx filter between Tx path (PA=power amplifier) and antenna for signalsto be sent.

[0002] Since the communication terminal device must be able tosimultaneously transmit and receive, each of the two filters must beable to suppress a signal lying in the other frequency band well.Typical values that are required in such wireless communication systems,for example for the suppression of the Tx band by the Rx filter, lie inthe range around 50 dB and above. At the same time, the respectivesignals dare experience only minimum losses when passing through thefilter in the respective frequency band. A typical value for a maximallytolerable attenuation of the Tx band in the Tx filter is 2 dB or better.

[0003] Known duplexers that meet these high demands made of the bandseparation (stop band suppression) and the insertion attenuation areconstructed, for example, of microwave ceramic filters. Given anadequate band spacing of TX and Rx band, surface-active wave filters(SAW filters) can also be employed. When, however, TX and Rx bands lieextremely close to one another, it is very difficult or even impossibleto simultaneously meet all demands with SAW filters by themselves. Oneexample of such a system is the American CDMA/TDMA-1900 (according toIS-95 or, respectively, IS-136) wherein transmission and reception bandare respectively 60 MHz wide and wherein a duplex spacing (constantspacing between transmission and reception signals) of 80 MHz isprovided. Given said bandwidth, a spacing of only 20 MHz thereby remainsbetween the two bands, this corresponding to approximately 10000 PPMgiven said frequency band. Within only 20 MHz, the respective filtermust thereby switch from the pass band with, for example, 2 dBattenuation into the stop band where, for example, said 50 dBattenuation is required. This requires a transmission behavior thatexhibits steep signal edges in the pass band. Since atemperature-dependent frequency drift of the filter as well asmanufacturing tolerances must also be additionally taken intoconsideration, it was hitherto not possible to construct such a duplexeronly on the basis of SAW filter technique. This required SAW filtersthat comprise a pass band with extremely steep signal edges.

[0004] It is therefore an object of the present invention to specify aduplexer for such communication systems that enables the employment ofSAW filter technique and, thus, allows for a miniaturization ofduplexers.

[0005] This object is achieved with a duplexer according to claim 1.Advantageous developments of the invention can be derived from thesubclaims.

[0006] The invention is based on the idea of composing at least one ofthe filters between antenna and RX path (reception filter) and betweenantenna and DX path (transmission filter) of at least two sub-filterswhose pass bands lie in mutually neighboring frequency sub-ranges of therequired transmission or, respectively, reception band and thereby coverthe entire band. These sub-filters fashioned as surface-active wavefilters can then be correspondingly narrower-band than the filterspreviously employed. Each of the two sub-filters, for example, then needonly cover half the bandwidth of the corresponding transmission or,respectively, reception band. The required overall width of thecorresponding transmission or, respectively, reception band derives fromthe addition of the two frequency sub-ranges.

[0007] A better optimization is possible for a narrow-band SAW filter,this particularly enabling the formation of steeper signal edges in thepass band. A better separation of the bands is already possible withonly one signal edge improved in steepness, insofar as this delimits thecorresponding transmission or reception band from the neighboringreception or, respectively, transmission band. The split filter can thusbe fashioned as SAW filter, which was hitherto not possible because ofthe signal edges that could not be set steeply enough and because of theslight band spacing. With the steeper signal edges of the sub-filters,the invention also enables a better suppression of the respectivelyother band (transmission or, respectively, reception band) of 50 dB andmore. Over and above this, the matching required for setting thefrequency position given traditional duplexers of microwave ceramic(MWK) is eliminated given the employment of SAW filters.

[0008] Preferably, both input filters as well as output filters arefashioned as split surface wave filters. The inventive duplexers thuscompletely composed of SAW filters, so that the advantages of SAWfilters compared to traditional MWK filters or, respectively, duplexerscan be completely exploited. In particular, a further miniaturization ispossible with a duplexer composed only of SAW filters, this enabling afurther miniaturization of the corresponding terminal devices whereinthe inventive duplexer is to be employed. Since sub-filters forfrequency sub-ranges that are narrower then the overall band areprovided in the transmission and in the reception band, a suppression ofthe respectively other band or, respectively, frequency sub-range ispossible in a simpler way. When, for example, input and output filtershaving a bandwidth of 60 MHz were employed for the American CDMA-1900system, then a maximum spacing of 20 MHz remained given a duplex spacingof 80 MHz between the two bands (frequency ranges). Inventively, aspacing of 50 MHz is now possible with input and output filters splitinto at least two sub-filters. Even given non-optimum signal edges ofthe corresponding pass bands, a better suppression of the respectivelyother band can thus be achieved. Due to the structuring only with SAWfilters, a one-chip solution for all filters of the duplexer alsobecomes possible.

[0009] In the inventive duplexer, moreover, switches can be provided forswitching between the sub-filters and, thus, for switching between thefrequency sub-ranges. Due to the provision of a switch, respectivelyonly one of the sub-filters of the split SAW filter is always connectedto the antenna, so that the other sub-filter or sub-filters do notdisturb the properties of the “active” sub-filter. It is thus alsopossible to optimize the sub-filters independently of one another to asuitable frequency position and a suitable edge steepness. When inputand output filters are fashioned as split SAW filters, then a sub-filterof the output filter is allocated to each sub-filter of the inputfilter, these together forming a sub-filter pair. With the assistance ofa shared switch or two individual switches, a switch can then besynchronously undertaken from an active to a previously passive, furthersub-filter pair. The sub-filter pairs are thereby allocated such to thefrequency sub-ranges that the duplexer spacing is adhered to. Usually,the frequency positions of the sub-filters in the input filter and inthe output filter are thereby respectively shifted by the same amount.This shift always ensues in pairs.

[0010] In another advantageous development, the duplexer is fashionedfor transmission and reception within at least two differentcommunication systems that use different frequency bands. This isachieved in a simple way in that the plurality input and output filtersand the appertaining switches are correspondingly multiplied. A separateset of input and output filters and the appertaining switches istherefore provided for each communication system for which the inventiveduplexer is designed. For example, terminal devices that are providedfor utilization in two different communication systems (dual band cellphones) are already known and employing separate duplexers for each ofthe systems is already known. Inventively, it is also possible to designone duplexer for more than two communication systems.

[0011] In a further development of the invention, one switch can switchboth between sub-filter pairs within a communication system as well asbetween sub-filter pairs that belong to two different communicationsystems. The duplex spacing can thereby also vary and, thus, so can thespacing between the frequency sub-ranges of the sub-filter pairs. Whenthe two communication systems are present parallel to one another andexhibits different degrees of coverage, a better network coverage isthen possible for a communication terminal device using the inventiveduplexer. When different communication systems are used in differentcountries, then a correspondingly equipped communication terminal devicecan be used in both systems in cross-border fashion. A common advantageis thereby always that only one duplexer is required for the differentcommunication system. A splitting of the input and/or output filtersinto two or even more sub-filters can thereby be undertaken in bothcommunication system systems. However, it si also possible that onecommunication system exhibits an adequately high duplex spacing that canbe realized with the assistance of respectively one SAW filter for inputand output filter. In combination with a communication system thatcomprises split input and/or output filters, a switching possibilitybetween at least three pairs of filters, whereof at least two sub-filterpairs are on an SAW base, thus derives for an inventive duplexer.

[0012] Preferably, all input and output filters of the duplexer and,potentially, the switches in addition thereto are arranged in a commonhousing or at least on a common module. This is easier to handle for themanufacturer of the terminal device and can be more simply optimized forterms of its properties.

[0013] Preferably, an inventive duplexer is constructed only of surfacewave filters for the reception band and the transmission band, all ofthese being integrated on a shared piezoelectric substrate or beingarranged on two substrates. Due to the high integration density that isthereby possible, the highest degree of miniaturization for the duplexercan thus be achieved with the first-cited embodiment. On a sharedsubstrate, further, the shared employment of other circuit and networkcomponents is also possible for the different filters, this yielding afurther enhancement of the integration density. Further, a simplifiedadaptation of the filters to one another and to a network is alsopossible on the common substrate.

[0014] It is also possible to integrate all filters and sub-filterstogether with a potentially required matching network of passivecomponents and the switches on a shared module. This also simplifies themanipulation and simplifies the employment since the manufacturers ofthe terminal devices need process only one module.

[0015] Lithium tantalate red y having a section angle of 35 through 46°(LT35-46) is preferred for the surface wave filter and SAW sub-filters.This material has an especially good temperature response with which atransmission behavior having narrow band widths and steep signal edgescan be set.

[0016] Since, due to the higher spacing between the frequencysub-ranges, signal edges that are not as steep also lead to the desireddecoupling between transmission and reception band given the inventiveduplexer, the employment of lithium niobate red y having a section angleof 60-70° (LN60-70) and, in particular, close to 64° (LT64) isfundamentally also possible. Even lower insertion attenuations can thusbe achieved compared to lithium tantalate. This can be particularlyadvantageous given the employment of lithium niobate for the outputfilters, since a high transmission power is desired particularly given acommunication terminal device, a low insertion attenuation beinginternally required for this. Given an unchanging transmission power, alower insertion attenuation results in a lower power consumption.

[0017] It is also possible to provide SAW filters for input and outputfilters that are constructed on different substrate materials. Thecombination lithium niobate for the output filter and lithium tantalatefor the input filter is thereby preferred.

[0018] In order to achieve the good filter properties, the electrodematerial is preferably correspondingly power-resistant. Electrodes thatcomprise the following layers of material or, respectively, a sandwichstructure having the following combinations of material layers aretherefore well-suited: aluminum and copper layers, aluminum andmagnesium layers or aluminum/copper and copper or magnesium layers.

[0019] An approved power compatibility is also achieved when the layercomprising titanium is provided between electrode material andsubstrate, particularly a titanium layer.

[0020] The surface wave filters of the inventive duplexer are preferablyfashioned as reactance filters, with which the required, high insertionattenuation can be easily achieved, particularly at the output filter.

[0021] The invention is explained in greater detail below on the basisof exemplary embodiments and the appertaining seven figures.

[0022]FIG. 1 shows the position and arrangement of transmission andreception band.

[0023]FIG. 2 shows a real filter curve.

[0024]FIG. 3 shows the arrangement and position of frequency sub-rangesaccording to the invention.

[0025]FIGS. 4 through 6 show various integration units of a duplexertogether with periphery.

[0026]FIG. 7 shows an exemplary interconnection of one-port resonatorsto form a reactance filter.

[0027] In a schematic illustration, FIG. 1 shows the arrangement andposition of transmission band TX and reception band RX of the AmericanCDMA-1900 system. The transmission band TX extends from 1850 through1910 MHz and is thus 60 MHz wide. The reception band RX extends from1930 through 1990 MHz and thus likewise has a width of 60 MHz. Acommunication connection—as viewed proceeding from the communicationterminal device—uses, for example, a transmission frequency fxT thatlies in the transmission band TX and, simultaneously therewith, uses areception frequency fxR in the reception band RX. The spacing betweensxT and sxR is what is referred to as the duplex spacing dA and amountsto 80 MHz for said CDMA system. For a communication connection withinthis system, all frequency pairs having the duplex spacing 80 MHz aresuitable. The spacing BA between transmission TX and reception band RXamounts to 20 MHz.

[0028] In a schematic illustration, FIG. 2 shows a possible pass curveof a filter with required bandwidth entered therebelow. Here thetransmission band TX. What is decisive for the filter quality is, inparticular, the insertion attenuation ED. Within the corresponding band,that is the greatest spacing from the broken-line zero line for zeroattenuation relative to the pass curve. Usually, the pass range is alsowider then the required frequency range of the respective band, sincethe signal edges of a filter cannot be vertically set in the pass band.Given the pass curve shown in the figure for a transmission filter, theright-hand edge F_(re) is critical, this limiting the passband relativeto the neighboring frequency range of the reception band RX. This edgemust be steep enough so that the input filter here exhibits anadequately low sensitivity or, respectively, an adequately high stopband suppression SU in the region of the reception band RX. For acorresponding input filter, the left-hand edge F_(li) of the passbandwill be critical, this limiting the reception band RX relative to thetransmission band TX.

[0029]FIG. 3 shows how the transmission and reception ranges TX, RX areinventively split into respectively two frequency sub-ranges having whatis here an identical bandwidth. A frequency sub-range RX1, RX2 of thereception band is respectively thereby allocated such to a frequencysub-range TX1, TX2 of the transmission band that the duplex spacing DAcan be adhered to. For example, a transmission frequency fXT has areception frequency fxR allocated to it in the required duplex spacingDA of, for example, 80 MHz. Whereas the spacing BA between transmissionand reception band in known duplexers corresponds to the spacingf1R−f3T, it amounts to f1R−f2T=f2R−f3T=50 MHz (for said CDMA system)given the inventively split transmission or, respectively, receptionbands or, respectively, appertaining filters. The filters belonging tothe corresponding frequency sub-ranges exhibit a passband in thecorresponding frequency sub-ranges. Due to the higher band spacing BA,however, filters having less steep edges can be selected for thispurpose, these nonetheless achieving the required stop band suppressionSU of, typically, 50 dB.

[0030] In addition to the division of transmission or reception bandinto two frequency sub-ranges Tx1, Tx2; Rx1, Rx2 shown here, of course,it is also possible to divide the corresponding bands into three andmore frequency sub-ranges, whereby a separate sub-filter is provided foreach frequency sub-range.

[0031] In a schematic illustration, FIG. 4 shows a duplexer composed offour sub-filters FR1, FR2, FT1, FT2 together with the interconnectionthereof to an antenna A and the appertaining transmission path PA andreception path LNA. Both the input filter as well as the output filterare fashioned as split surface wave filters each having two sub-filters.The input filter comprises the sub-filters FR1 and FR2, whereas theoutput filter comprises the sub-filters FT1 and FT2. A switch S that canswitch between two sub-filter pairs FT1/FR1 and FT2/FR2 is arrangedbetween the antenna A and the duplexer composed of the four sub-filters.A sub-filter pair thereby respectively comprises a filter composed ofinput and output filter, for example the pair FR1/FT1 or FR1/FT2.Further switches S′ S″ connect, for example, the components of thereception path LNA to the input filter, whereby the switch S′ switchesbetween the sub-filters of the input filter. Correspondingly, the switchS″ switches between the various output filters FT1, and FT2 that areoptionally connected to the components of the transmission path PA. Thebroken illustrates a module M1 on which the four sub-filters areintegrated. The matching network composed, for example, of passivecomponents such as resistors, capacitors and inductances or striplines(not shown in the Figure) is realized outside the module, just like theswitches S.

[0032]FIG. 5 shows a corresponding arrangement wherein, however, thematching network is also additionally integrated on an enlarged moduleM2 in addition to the sub-filters.

[0033] An even more highly integrated module M3 is shown in FIG. 6. Thismodule M3 also comprises the matching network and the switches S inaddition to comprising the sub-filters.

[0034] In a schematic illustration, FIG. 7 shows a circuit arrangementfor a reactance filter composed of surface wave one-port resonators. AnSAW one-port resonator is constructed on a piezo-electric substrate 1and comprises an interdigital transducer IDT provided with two terminalsthat is arranged between two reflectors Ref. For a simple reactancefilter, at least two such one-port resonators are then interconnectedsuch that at least one of the resonators is serially arranged betweeninput ES and output AS and at least one of the resonators is connectedto the ground in a parallel branch. Together with a neighboring,parallel resonator R1P, a serial resonator, for example R1S, forms abasic element of a reactance filter. A reactance filter, however, ispreferably composed of a plurality of series-connected basic elements,for example of three basic elements as shown in the Figure. In theexemplary embodiment, the resonators R2S and R1P as well as R2S and R2Pform two further basic elements. Within a basic element, the resonantfrequencies of parallel and serial resonator are shifted such relativeto one another that the anti-resonant frequency of the serial resonatorcomes to lie exactly on the resonant frequency of the parallelresonator. The filter thereby exhibits a pass behavior with a pass bandthat exhibits an especially low insertion attenuation of, for example, 2dB and less.

[0035] The exemplary embodiments only stand by way of example forpossible embodiments of the invention. The invention, however, is notlimited to the exemplary embodiments and can comprise further variationsthat are not shown.

LIST OF REFERENCE CHARACTERS

[0036] Tx transmission band

[0037] Rx reception band

[0038] SU stop band suppression

[0039] ED insertion attenuation

[0040] fxT transmission frequency

[0041] fxR reception frequency

[0042] F_(li) F_(re) left and right edge of the pass band

[0043] Tx1, Tx2 frequency sub-ranges of the transmission band

[0044] Rx1, Rx2 frequency sub-ranges of the reception band

[0045] FR1, FR2 sub-filters of the split input filter

[0046] FT1, FT2 sub-filters of the split output filter

[0047] A antenna

[0048] S, S′, S″ switches

[0049] LNA low noise amplifier of the Rx path

[0050] PA power amplifier of the Tx path

[0051] M1, M2, M3 modules

[0052] R1S, R2S one-port resonators in the serial branch

[0053] R1P, R2P one-port resonators in the parallel branch

[0054] ES serial branch input

[0055] AS serial branch output

[0056] Ref reflector

[0057] IDT interdigital transducer

Patent claims
 1. Duplexer for a wireless communication system comprisinga transmission and a reception band (Tx, Rx), whereby different filtersare provided as input and output filters for the transmission and thereception band or, respectively, for transmission and reception signal;whereby input and output filter is fashioned as split surface wavefilter with at least respectively two sub-filters (FR1, FR2; FT1, FT2)covering neighboring frequency sub-bands of the transmission or,respectively, reception band; whereby a sub-filter (FR1) of the outputfilter is allocated such to a respective sub-filter (FT1) of the inputfilter that the duplexer spacing between transmission and receptionsignal is adhered to for all sub-filter pairs; whereby at least onefurther pair of filters is provided that represent input and outputfilters for a further communication system and exhibit a differentduplexer spacing; whereby switch elements are provided for switchingbetween sub-filter pairs and the further pair of filters.
 2. Duplexeraccording to claim 1, fashioned for sending and receiving in at leasttwo communication systems using different frequency bands, whereby splitsurface wave filters (FR1, FR2; FT1, FT2) serving as input and/or outputfilters are provided for the at least two frequency bands (Rx, Tx). 3.Duplexer according to one of the claims 1 or 2, whereby the input and/oroutput filters fashioned as split surface wave filters (FR1, FR2; FT1,FT2) comprise more than two sub-filters.
 4. Duplexer according to one ofthe claims 1-3, whereby all input and output filters and the switch orswitches (S) are arranged in a common housing.
 5. Duplexer according toclaim 3, whereby all input and output filters are formed on a commonpiezoelectric substrate.
 6. Duplexer according to claim 3 or 4, wherebya matching network and all existing filters or sub-filters areintegrated on a common module (M).
 7. Duplexer according to one of theclaims 4-6, whereby the switches (S) are arranged on a common substratewith the filters.
 8. Duplexer according to one of the claims 1-7,whereby the substrate material of at least one [. . . ] of input andoutput filter comprises lithium tantalate red y with a section angle of35 through 44°—LT35-44.
 9. Duplexer according to one of the claims 1-7,whereby the substrate material of at least one [. . . ] of input andoutput filter comprises lithium tantalate red y with a section angle of60 through 70°—LT60-70—and, in particular, having a section angle closeto 64°—LN64.
 10. Duplexer according to claim 8 and 9, whereby thesubstrate material for the input filter comprises LN64, and thesubstrate material for the output filter comprises LT35-44.
 11. Duplexeraccording to one of the claims 1-10, whereby the surface wave filterscomprise electrode structures that are composed of a material selectedfrom aluminum and copper, aluminum and magnesium, aluminum andmagnesium, aluminum/copper and magnesium.
 12. Duplexer according toclaim 11, whereby the electrode structures comprise a titanium layer.13. Duplexer according to one of the claims 1-12, whereby the surfacewave filter or filters are fashioned as reactance filters. 14.Employment of the duplexer according to one of the preceding claims in atransceiver.